1. Field of the Invention
This invention relates to a fluorescent display tube, and more particularly to a fluorescent display tube of the type that luminous display is observed through a substrate which can carry out luminous display easily observable and without any display defect and simplify the manufacturing process.
2. Description of the Prior Art
A conventional fluorescent display tube is generally constructed in a manner such that luminous display in the form of a letter, a figure or the like obtained by impinging electrons emitted from a heated cathode on an anode having a fluorescent layer deposited thereon is observed through the cathode. However, the fluorescent display tube of such type has a disadvantage that it is difficult to provide luminous display with a good visibility and a wide visual angle.
In order to eliminate such disadvantage, a fluorescent display tube of the type that luminous display is observed through a substrate has been recently proposed. In a fluorescent display tube of this type, a substrate on which anode sections are to be formed is made of a light permeable material such as glass or the like and an anode is formed of a transparent conductive material such as SnO.sub.2, In.sub.2 O.sub.3 --SnO.sub.2 composite referred to as "ITO" so that luminous display may be observed through the anode and substrate.
A conventional fluorescent display tube of such type will be hereinafter described in detail with reference to FIG. 1.
A fluorescent display tube shown in FIG. 1 includes a substrate 1 made of a light permeable insulating material such as a sheet glass, on which wiring conductors 2 and anode conductors 4 each formed of a transparent conductive material are deposited. The substrate 1 also has insulating layers 3 of a light impermeable property applied thereon except the portions thereof corresponding to display pattern sections. Each of the anode conductors 4 has a fluorescent layer 5 deposited on the exposed surface thereof which is not covered by the insulating layer 3, to thereby form an anode 6. Above the anodes 6, filamentary cathodes 7 are stretched so as to oppose to the corresponding anodes. The fluorescent display tube also includes a mesh-like control electrode 8 disposed between the anodes 6 and the cathodes 7. These electrode elements are received in an envelope which is formed by hermetically sealing a casing 9 on the periphery of the substrate 1 and is highly evacuated. The wiring conductors 2 are led out from the casing 9 to be connected to corresponding external terminals 10.
The fluorescent display tube of FIG. 1 adapted to observe luminous display of the anodes 6 through the substrate 1 has an advantage of accomplishing luminous display easily observable because a wide visual angle is obtained and the internal structure is out of sight by the substrate.
Recently, a fluorescent display tube has been extensively used in various environments with the extension of use. For example, a fluorescent display tube incorporated in display systems for vehicles is used in a light environment as well. However, the conventional fluorescent display tube of the type that display is observed through a substrate as mentioned above is not suitable for use in such environment because it cannot form a distinct contrast between light emitting anodes and non-light emitting anodes. In addition, in the conventional fluorescent display tube of this type, the wiring conductors are formed of metal oxide. However, metal oxide is not optimum as a material for the wiring conductor, because the wiring conductor formed of this material is apt to increase in resistance.